Acid enhanced zipping effect to densify MWCNT packing for multifunctional MWCNT films with ultra-high electrical conductivity

The outstanding electrical and mechanical properties remain elusive on macroscopic carbon nanotube (CNT) films because of the difficult material process, which limits their wide practical applications. Herein, we report high-performance multifunctional MWCNT films that possess the specific electrical conductivity of metals as well as high strength. These MWCNT films were synthesized by a floating chemical vapor deposition method, purified at high temperature and treated with concentrated HCl, and then densified due to the developed chlorosulfonic acid-enhanced zipping effect. These large scalable films exhibit high electromagnetic interference shielding efficiency, high thermoelectric power factor, and high ampacity because of the densely packed crystalline structure of MWCNTs, which are promising for practical applications.

error bars as shown in the following Table S1 since the pristine-MWCNT films were compressed before used. These films had a relatively flat surface as shown in Figure S6. The voided area of the CNT cylinder was included in the cross-sectional area calculation. The obtained MWCNT films contained CNTs and irregular nanoparticles.
Nearly 94% MWCNTs had either double walls or triple walls with a broad distribution in diameter of 1-5 nm as counted from the transmission electron microscope images shown in Figure S1. The diameter for most nanoparticles was in the range of 5-10 nm. Raman spectra indicated that the MWCNTs preferred to align along the direction parallel to the winding direction with an IG ∥ /IG ⊥ ratio of 1.87 ( Figure   S3). The high IG/ID ratio of 16.3 also demonstrated the high quality of the synthesized MWCNT films. After annealing, the IG ∥ /IG ⊥ and IG/ID ratios became 1.95 and 18.38, respectively. ' Figure S4 Thermal gravimetric curves of pristine MWCNT and annealed-MWCNT film in the air atmosphere.
Pristine-MWCNTs showed two weight losses at ~350 o C and 450 o C, respectively. The first weight loss of the pristine-MWCNTs was attributed to the degradation of amorphous carbon. 5 And the second weight loss of the pristine-MWCNTs was due to the oxidation of carbon nanotubes (CNTs). 6 While the annealed-MWCNTs had only one weight loss at 450 o C due to the oxidation of CNTs. 6 According to the distribution of the number of walls in Figure S1, an average diameter (DCNT) was 2.85 nm.
Figure S5 X-ray photoelectron spectroscopy spectrum of Fe for pristine MWCNT film.
A peak appeared at 706 eV in the X-ray photoelectron spectroscopy spectrum, which was assigned to the iron or iron carbide nanoparticles. 7 ( Figure S4) Some of the iron nanoparticles might be oxidized in the air to reduce the electrical conductivity of the MWCNT films. 4  The thicknesses of pristine-MWCNT, annealed-MWCNT, HCl-MWCNT and CSA-MWCNT were 8.340.92 m, 7.070.35 m, 6.640.19 m, 0.640.05 m, respectively, as shown in Figure S7. The cross-sectional areas were 0.04170.0046 mm 2 , 0.03530.0017 mm 2 , 0.03320.0010 mm 2 , and 0.00320.0002 mm 2 for as-synthesized MWCNT films, annealed-MWCNT films, HCl-MWCNT films and CSA-MWCNT films, respectively. This cross-sectional area was pretty uniform as shown in the following Table S1 since the pristine-MWCNT films were compressed before used. These films had a relatively flat surface as shown in Figure S6.    According to the distribution of the number of walls in Figure S1, an average diameter (DCNT) of 2.85 nm for MWCNTs was used in the calculation as shown in Figure S12. PF  values were also measured for the MWCNT films which were in the     The electrical conductivity of the film was measured by commercial equipment (NETZSCH SBA-458, Germany) with a four-probe method. All the measurements were performed under Ar protection at room temperature and the MWCNT film samples were cut into strips with a length of 20 mm and a width of 5 mm. The distance between the electrodes could be seen in Figure   S18, which is about 3-8.5 mm.
The electrodes were made of rhodium which contacted directly with the samples. The pressure was applied to ensure good contact between the sample and the rhodium electrodes by a pressure disk and the knurled nuts as shown in Figure S18. The electrical contact resistance between the electrodes and the samples was eliminated by the four-probe method. Table S1 The electrical conductivities and thicknesses of CNT films Table S2 The maximum σ ∥ of CSA-MWCNT films was compared with that of CNT only and CNT composite films reported in the literature.     The optical image of the pristine-MWCNT film has been added in Figure  S1 in the supplementary information. Typically, the film had a length of 25 cm and a width of 28 cm. The sizes of MWCNT films used in this work have been summarized in Table S9.